Twitter says it is blocking accounts related to "graphic imagery" after the Islamic State released a video that shows the beheading of American photojournalist James Foley, who was kidnapped in Syria nearly two years ago.

Twitter also suspended an account belonging to the Islamists' media arm. The Islamic State has turned instead to opening accounts on another social media platform called Diaspora, the BBC reports.

Costolo's tweet followed online calls to not share the video or images of the purported beheading, using hashtag #ISISmediablackout. ISIS refers to the Islamic State of Iraq and the Levant, one of the former names of the extremist Islamist militants.

Twitter user @LibyaLIberty appeared to start the hashtag, posting on Tuesday, "From here on out, I won't share any photo or video of violence intentionally recorded & released by ISIS for propaganda. #ISISmediaBlackout"